pkg/boot/linux.go: Decompress kernel before doing kexec

[CodingVoid]

It is possible/likely that the kernel is compressed. In order for kexec to actually find the magic value for aarch64 Linux kernels it needs to be decompressed before refering it to the kexec syscall.

Test: aarch64 Fedora 36 Server image can now be started via localboot.

[hugelgupf]

A kernel should decompress itself on kexec, shouldn't it? What kind of compression is on this thing? (Is that a feature not available on aarch64?)

u-root's kexec is in use in memory-constrained environments sometimes, so we'd likely prefer if this was either behind a flag or could be avoided altogether due to its use of memory.

[CodingVoid]

On aarch64 kexec does not decompress the kernel (that may not be true for the new zimage in Linux though). As far as I know that is only happening on x86 (bzImage). That is actually what the kexec u-root command already does (so if I do "kexec ..." in u-root it works but not with the localboot cmd). No Idea why there are different code paths for kexec and kexec localboot cmd in the first place, but I guess that is something for another patch.
kexec u-root cmd decompression:

u-root/pkg/boot/linux.go

Line 284 in 585ee83

k, err := copyToFileIfNotRegular(util.TryGzipFilter(li.Kernel), verbose)

I used a aarch64 fedora server image for testing.

[hugelgupf]

On aarch64 kexec does not decompress the kernel (that may not be true for the new zimage in Linux though). As far as I know that is only happening on x86 (bzImage). That is actually what the kexec u-root command already does (so if I do "kexec ..." in u-root it works but not with the localboot cmd). No Idea why there are different code paths for kexec and kexec localboot cmd in the first place, but I guess that is something for another patch. kexec u-root cmd decompression:

u-root/pkg/boot/linux.go

Line 284 in 585ee83

k, err := copyToFileIfNotRegular(util.TryGzipFilter(li.Kernel), verbose)

I used a aarch64 fedora server image for testing.

Oh this is for jsonboot. I didn't even notice that. It probably ultimately shouldn't be a different codepath, but jsonboot doesn't yet.

[hugelgupf]

Ultimately, IMO any remaining localboot functionality should merge into the boot command (which uses the code path kexec uses), and localboot should be deleted.

[hugelgupf]

Have you looked into the failing tests?

[hugelgupf]

pkg/boot/linux_test.go:144:12: undefined: copyToFileIfNotRegular

[codecov]

Codecov Report

Patch coverage is 42.85% of modified lines.

Files Changed	Coverage
pkg/boot/jsonboot/bootconfig.go	0.00%
pkg/boot/linux.go	100.00%

📢 Thoughts on this report? Let us know!.